The overall purpose of this project is to understand how viruses interact with their hosts in regulating viral development. The virus used in these studies is the temperate coli-phage lambda. We have been concentrating our effort on studying the nature of the specific host virus interactions of one class of phage regulatory functions, a class of regulatory functions represented by the N gene product of lambda. We have isolated a number of E. coli mutants which block lambda growth by reducing or eliminating the activity of the N gene product. Thus far, we have been able to determine that bacterial functions coded for by three distinct genetic loci are directly involved in N activity. The N function stimulates gene expression by permitting initiated RNA transcripts to proceed through specific termination sequences. In a series of experiments, using one of these bacterial mutants, we have been able to define three types of phage promoters depending on their ability to utilize the N of lambda and the N of phage P22 to effect antitermination: 1) Type 1 can utilize both N of lambda and P22. 2) Type 2 can utilize only the N of P22. 3) Type 3 can utilize neither N product.